amu and g are units of mass
molecules and moles are units of amount
amu is used when you're working on the order of individual molecules/atoms. g is used when you're working on the order of moles of molecules/atoms. It's just a matter of scale.
amu stands for atomic mass units. Some perspective: the mass of one atom of carbon is ~1.99x10
-23 g. (really small number). To manipulate numbers in those units is awkward and error prone. Thus, we came up with the amu to simplify things. 1 amu = ~1.7x10
-24 g. With this convenient unit, we can now say the mass of one atom of carbon is 12 amu. It works out that amus and grams are related. one carbon atom has a mass of 12 amu. One mole of carbon atoms has a mass of 12 g. Wikipedia does a pretty good job here.
http://en.wikipedia.org/wiki/Atomic_mass_unitNow, as to your questions. My first suggestion - especially because this is related to the topic of your post -
always include units in your calculations. And pay attention to your scale. For your first question, you start your calculations with the number 12. No good. Gotta include units. They're often given in the question. 12 carbon atoms. This also tells us the scale we're working on - atoms. One carbon atom has an atomic mass of 12.011 amu. Use this conversion factor.
(12 carbon atoms)*(
12.011 amu/
1 carbon atom) = 144.132 amu. Since we're already on the order of atoms, we don't have to divide by Avogadro's number here. Same analysis for oxygen.
B - good. but be careful with your wording. O is not diatomic. Molecular oxygen (O
2) is.
C - you should start with what is given in the problem: 1 mole. amu is not the unit for amount. atoms is the unit for amount. amu is a unit of mass.
E - molar mass is not needed here. What is the relationship between # of molecules and moles?
F - again, mass is not needed.
G - molar mass of a compound is the sum of the molar mass of the individual atoms. The oxygen molecule has a molar mass of 32 because the sum of the molar mass of the individual atoms = 16 + 16 = 32.